This invention relates to an improvement in the continuous or semicontinuous casting of molten metal in an alternating current electromagnetic field to control the shape of the solidifying metal.
Ingots or billets which have been continuously or semi-continuously D.C. cast in conventional open-ended tubular molds are usually characterized by a surface roughened by defects, such as cold folds, liquations, hot tears and the like, which result primarily from contact between the mold and the solidifying embryonic metallic shell. Moreover, conventionally D.C. cast ingot and billet are also characterized by a surface zone which has considerable alloy segregation due to the initial cooling of the molten surface from contact with the mold, reheating of the metal surface after mold contact and then final cooling of the metal surface from the direct application of coolant. Subsequent fabrication steps, such as rolling, extruding, forging and the like, usually require the scalping of the ingot or billet prior to working to remove both the surface defects and the alloy impoverished zone adjacent the surface.
In electromagnetic casting, there is no contact with the embryonic metallic shell during solidification and due to this lack of contact, most surface defects are eliminated. Moreover, due to the lack of contact between the embryonic metal surface and a mold surface, there is usually no cooling, heating, then cooling of the metal surface which causes the formation of the alloy impoverished zone adjacent the surface. As a result, electromagnetically cast metal has an essentially homogeneous composition throughout the entire cross section thereof. Because the electromagnetically cast metal surface is smooth and has essentially no alloy constituent segregation, there is usually no need to scalp the electromagnetically cast material prior to fabrication. Additionally, due to the homogeneous composition and structure, there is considerably less edge cracking during hot rolling so that less edge trimming is necessary after rolling.
The electromagnetic field utilized in electromagnetic casting generates forces normal to the surface of the molten metal which control the shape of the molten metal during solidification. The field is produced by a ring-type inductor and when molten metal is fed to the inner peripheral area of the inductor, the interaction of the electromagnetic field with the eddy currents induced in the molten metal generates the electromagnetic forces which control the cross-sectional shape of the solidifying metal to the same general shape as the inductor. The radial force components generated by the electromagnetic field prevent any significant lateral movement of molten metal and thus no contact between the molten metal and the inductor occurs. With the application of coolant, the molten metal solidifies in the shape induced by the electromagnetic field. A high frequency electrical power source (e.g., 500-3000 cps) is usually employed in electromagnetic casting because at the high frequencies, the induced currents in the molten metal concentrate at the surface of the molten metal (commonly termed "skin effect") so there is very little turbulence in the molten metal.
The principles of electromagnetic casting are basically the same principles as those of electromagnetic levitation and zone refining, which are well described in the literature, e.g., see U.S. Pat. No. 2,686,864 (Wroughton et al) and the Journal of Metals, Vol. 4, pp 1286-1288 (1952). Getselev and his coworkers developed a practical electromagnetic apparatus for casting large commercial-sized ingots and billet based on these principles. Their design was first described in U.S.S.R. Inventor Certificate No. 233,186 (issued Dec. 18, 1968) and various modifications of the basic design are shown in U.S. Pat. Nos. 3,467,166; 3,605,865; 3,646,988; 3,702,155 and 3,773,101. For additional descriptions of the electromagnetic casting unit and process developed by Getselev et al, see Tsvetnye Met, August 1970, Vol. 43, (8), 64-65 and the Journal of Metals, Vol. 8, October 1971, pp. 38-39. See also U.S. Pat. No. 3,741,280.
Two different ring-type electromagnetic inductors have been described by Getselev and both usually require continual contact with coolant to control the temperature of the inductor. The first type is a hollow inductor internally cooled with a suitable fluid such as water. This type of inductor is difficult and expensive to fabricate and maintain, but nonetheless, it is an effective inductor when properly used. The other type inductor described by Getselev involves a solid inductor disposed within a coolant chamber of the water jacket, preferably in an area where there is a high water velocity to maintain appropriate heat transfer rates. However, this latter method reduces the electromagnetic efficiency due to the increased distance required between the inductor and the molten metal surfaces being controlled. Moreover, the nonmetallic members adjacent the metal being cast may be damaged by metal spills and the like and are also subject to thermal and mechanical distortion.
The radial component of the electromagnetic pressure against the molten metal column generally must be equal to the hydrostatic pressure of the molten metal being shaped. To compensate for the gradually lower hydrostatic pressure of the molten metal column progressing toward the upper portions thereof, a shield or screen is preferably positioned between the inductor and the top of the molten metal column to attenuate the electromagnetic field generated by the inductor and thereby gradually reduce the radial forces acting on the molten metal toward the top of the column (see U.S. Pat. No. 3,467,166 -- Getselev et al). By this means, the molten metal surface can be maintained relatively straight in the vertical direction and the curvature of the top corners of the molten metal column can be maintained relatively small. Without the electromagnetic screen, the curvature radius of the upper corner or corners of the shaped molten metal can become so large that the curved top surface of the molten metal column intersects with the solidification zone causing severe surface waves and other defects. However, the placement of the electromagnetic shield between the inductor and the molten metal surface being controlled requires positioning the inductor farther away from the molten metal surface, which increases electrical power requirements. Additionally, the electromagnetic shield can consume up to 30% or more of the electrical power supplied to the inductor.
Getselev found that large masses of metallic materials are not desired in the immediate vicinity of the electromagnetic inductor because a large metallic mass interferes with the electromagnetic field employed to control the shape of the solidifying ingot and can consume large amounts of energy. For this reason, the water jacket and other components, except for the electromagnetic shield and inductor, are formed of nonmetallic, non-conducting materials, such as micarta, epoxy-bonded fiberglass and the like. However, the nonmetallic, nonconductive members of the water jacket were found to be subject to mechanical and thermal distortions which interfered with the even application of coolant onto the solidifying ingot. Uneven coolant application detrimentally affects surface cooling and severely interferes with the uniform solidification patterns in the metal necessary for high quality ingot or billet. As a result, the nonmetallic parts of the coolant distribution system normally need frequent maintenance or replacement to maintain the appropriate distribution of coolant around the solidifying metal.
It is against the background that the present invention was developed.